1. Field of the Invention
This invention relates in general to tension leg platforms for offshore drilling, and in particular to a top connector for connecting the upper end of a tendon to the platform.
2. Description of the Prior Art
A TLP or tension leg platform is a type of offshore drilling and production structure. The platform floats and is secured to the sea floor by tendons. The tendons are large pipes, about 20 to 45 inches in diameter. After securing the tendons to the platform and to the sea floor, ballast water is pumped out to cause the platform to rise. This is resisted by the tendons, placing the tendons under tension load. The tendons under tension provide a stable platform for drilling and oil production.
U.S. Pat. No. 4,871,282, Charles E. Jennings, Oct. 3, 1989 and U.S. Pat. No. 5,020,942, Joseph W. Pallini, Jr., Jun. 4, 1991, describe top connectors for connecting the top of the tendon to the platform. In the devices shown in these patents, the upper end of the tendon has helical threads. The threaded section extends through a housing which has a conical bore. Segments, or dogs, will slide down the conical bore from a retracted position to an engaged position. The segments having mating threads which will engage the threads on the tendon. A cam ring causes the downward movement of the segments. The cam ring will also rotate the segments once in contact with the tendon to mesh the threads of the segments with the threads of the tendon.
While the devices of these patents are workable for certain platforms, disadvantages exist for some very large tension leg platforms. In one large platform being proposed, there will be no tendon tensioning, motion compensator or arresting devices.
The platform is large enough that relative motion between the platform and the tendons is small and slow. For this reason, the tendon handling system will support only the weight of the tendon plus a slight overpull.
In that system, the top connector will be required to dynamically lock off the platform to the tendon and also to accommodate upward relative motion of the tendon during downward movement of a platform. Trim adjustments must also be made over a period of several hours while the platform is adding ballast. The platform will move down relative to the tendons during the ballasting procedure. This requires that the segments be rotated to tighten the engaged segments during the ballasting procedure.
The rotation would be at a much higher speed than used with the configuration shown in the abovementioned patents. Also, this rotation would have to be performed over several hours. This would require a hydraulic system much more powerful than previously employed. Moreover, all of the top connectors must be actuated at the same time, rather than one per column as in the prior art tension leg platforms.